National Pipe Thread: The North American standard

The nominal size refers not to the outside of the pipe, nor even to the outside of the thread, but to its internal bore - the inside of the tube where the fluid flows. It makes sense when sizing pipes for flow rate but it makes things confusing when measuring for replacement parts.

Below is a summary of the common NPT thread sizes used with cast iron radiators and their corresponding actual outside diameters. The actual outside diameter is what you can measure when looking at a valve spud, a bushing or an opening in the radiator.

Nominal pipe size (NPT)

Threads per inch

Actual outside diameter, OD

inch

mm

​1⁄8"

27

0.405

10.3

​1⁄4"

18

0.540

13.7

​3⁄8"

18

0.675

17.1

​1⁄2"

14

0.840

21.3

​3⁄4"

14

1.050

26.7

1"

​11-1⁄2

1.315

33.4

​1-1⁄4"

​11-1⁄2

1.660

42.2

​1-1⁄2

​11-1⁄2

1.900

48.3

2"

​11-1⁄2

2.375

60.3

​2-1⁄2"

8

2.875

73.0

Right first time

We've made a handy PDF that you can print out at home to use as a gauge when measuring threads. Download our NPT Thread Size Template, print it onto US Letter paper at 100% scale and follow the instructions. It includes both circles that are sized for various NPT threads as well as a handy cut-off tape measure that wraps around the pipe if the ends aren't accessible.

The difference between NPT and BSP

NPT has flattened peaks and troughs where BSP is rounded. NPT has a thread angle of 60º where BSP is 55º.

At certain sizes, NPT and BSP share the same number of threads per inch - at ½" and ¾" - but at all other sizes the two are quite different. The table below lists the TPI for both NPT and BSP at sizes commonly used with domestic radiators.

Nominal pipe size

NPT

BSP

Threads per inch

Threads per inch

​1⁄8"

27

28

​1⁄4"

18

19

​3⁄8"

18

19

​1⁄2"

14

14

​3⁄4"

14

14

1"

​11-1⁄2

11

​1-1⁄4"

​11-1⁄2

11

​1-1⁄2

​11-1⁄2

11

2"

​11-1⁄2

11

​2-1⁄2"

8

11

The threads we use in our radiators

In brief: products for sale in North America ship with NPT threads; products for sale in Europe ship with BSP threads.

Our radiators are fitted with steel NPT bushings for the American market.

Our valves, for the US market, use NPT spuds and, where it's a compression fitting, are made for ½" US-standard pipe, with a ⅝" ferrule. Where the valve uses threaded connections, those are US-standard FIP (Female Iron Pipe).